The speaker considers the age-old question of whether the world will end in fire or in ice. This is similar to another age-old question: whether it would be preferable to freeze to death or burn to death. The speaker determines that either option would achieve its purpose sufficiently well.
The speaker brings us into the middle of an argument between people who think the world will come to a fiery end and people who think the world will freeze. He could be talking about the literal end of the world, but he's also talking about the power that human beings have to harm or "destroy" one another.